1. Field of the Invention
The present invention relates to a heating control device, a heating control method, and an image forming apparatus that uses the heating control apparatus that can detect an abnormality of an object to be heated while heating the object by electromagnetic induction.
2. Description of the Related Art
An image forming apparatus, such as a copier or a printer, that uses an electrophotography process is provided with a fixing device that heats and melts an unfixed image (for example, an unfixed toner image) formed on a recording sheet such as recording paper sheets etc. to fix the image.
One of such fixing devices heats a fixing belt, which is included in the fixing device, by electromagnetic induction.
The fixing belt of which substrate comprises a magnetic metal layer has endless structure and is rotated. This kind of fixing device has a coil that heats the fixing belt as an object to be heated, a pressure device that contacts with the fixing belt while giving pressure thereto to form a nip position, and a heating control device that controls electric power applied to the coil to control heat generation.
The heating control device generates alternating field by applying electric power to the coil, generates an eddy current in the substrate of the fixing belt by this alternating field, and heats the fixing belt with Joule heat generated by this eddy current.
Then, when a recording sheet on which an unfixed toner image is conveyed to the nip position, the heating control device heats and fixes the unfixed toner image onto the recording sheet by the heat of the fixing belt.
Incidentally, the fixing belt that generates heat by electromagnetic induction heating may be damaged by prolonged use. When the fixing belt is damaged, an unfixed toner image cannot be fixed completely.
For example, since the number of sheets processed by an image processing job (for example, a copy job) of an image forming apparatus may amount to 1000, when the copy job is executed with a damaged fixing belt, a large amount of imperfect copies will occur, which wastes recording sheets and toner.
Accordingly, a conventional heating control device is equipped with a means for detecting abnormalities of the fixing belt. The heating control device is provided with an antenna that generates voltage or current corresponding to a magnetic flux from the coil, and determines that the fixing belt is damaged when an output value of the antenna exceeds a predetermined threshold value. Then, when determining that the fixing belt is damaged, the heating control device makes the image forming apparatus interrupt a job under operating condition in order not to waste recording sheets and toner.
FIG. 10 is a graph showing an example of a relationship between the output value of the antenna (referred to as an antenna output, hereafter) and a percentage of an abnormality area in the fixing belt (a proportion of an abnormality area in one fixing belt) in the conventional heating control device.
In FIG. 10, a horizontal axis shows the percentage (%) of the abnormality area in the fixing belt, and the vertical axis shows the antenna output Va (V). The heating control device determines that the fixing belt is damaged when the antenna output Va exceeds the predetermined threshold value. Then, the conventional heating control device sets the abnormality detection threshold value for the antenna output Va according to an electric power set value showing the electric power applied to the coil. It should be noted that the abnormality detection threshold value for the antenna output Va is determined according to the antenna output Va at which the percentage of the abnormality area in the fixing belt becomes 25%.
As shown in FIG. 10, when the electric power set value is 1200 W (the maximum electric power; shown by a straight line 10c), the abnormality detection threshold value of the antenna output Va is set to the antenna output Va=0.30 V at which the percentage of the abnormality area in the fixing belt becomes 25%. Similarly, when the electric power set value is 600 W (a half of the maximum electric power; shown by a straight line 10b), the abnormality detection threshold value of the antenna output Va is set to the antenna output Va=0.15 V. When the electric power set value is 300 W (a quarter of the maximum electric power; shown by a straight line 10a), the abnormality detection threshold value is set to the antenna output Va=0.075 V.
Then, the heating control device compares the antenna output Va with the abnormality detection threshold value that has been set according to the electric power set value during a heating-fixing operation of the fixing device. Then, when the antenna output Va exceeds the abnormality detection threshold value concerned, the heating control device determines that the fixing belt is damaged (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2007-328159 (JP 2007-328159A)).
However, the antenna output Va increases with increasing the input voltage to the coil even if the electric power set value (i.e., the electric power) does not change. Therefore, when the abnormality detection threshold value of the antenna output Va is set according to only the electric power set value, variation of the input voltage changes a criterion of determination for the damage to the fixing belt.
Particularly, when an image forming apparatus is made universal and a range of a usable alternating voltage (an AC input voltage) spreads from the commercial voltage 100 V in Japan to the overseas commercial voltage 240 V, the criterion of determination for the damage to the fixing belt differs greatly.
Therefore, when the heating control device tries to determine damage to the fixing belt in consideration of the range of the AC input voltage, it is necessary to set the abnormality detection threshold value more highly than the case where the AC input voltage is 100 V. As a result, there is a problem that the damage to the fixing belt is not determined unless the percentage of the abnormality area in the fixing belt becomes too large, when the AC input voltage is 100 V.